Activated electrode for electric discharge lamp



od. 7, 195s 2,855,264

ACTIVATED ELECTRODE FOR ELECTRIC DISCHARGE LAMP Filed sept. 2o. 1955 nited States Patent ACTIVATEDELECTRODE FORE-ILECTRIC DISCHRGEL'AMP ',pour'lesfApplicationsde-.l-Elechictefet des Gaz V.Rares- .Etablissements Claude-Paz. &- Silva, Paris, France ApplicationV September 20, ;1'9'55,"Se`rillN0.i 535,331

.Claimsprority,.applicationvFx-ance September.22, .19.54

2 "Claims. (CH1516-8) This invention relates to processes `fortmanlifacturing electric discharge-lamps which vcontain anionizable-:dis

.charge atmosphere an'd Whichhave. at` least one-:activated a very high silica .contentand the ractivatedlectrode, or

electrodes, `of which comprise a refractory metal .mem-

.ben provided `with electronfemissive material, Vsaid electronemissive material being obtained ,fromasubstance .which has .a .melting .point highertthan .2500" .C...and1w.hich produces, during theformation ofthe electrode, `Aonlygproductsamong Whichthose which are not gaseous in the conditions of their productionhave .melting pointshigher than 2500 C., these .meltinglpoints being measured in .the conditions .to .whichfthe .substancefand products .are subjected when the lamp islbeing. manufactured.

In the above .definition .of .the substance, the .term formation.ofwtheI electrodes does not comprise the electric discharges to -Which `electrodes .are ,sometimes .-subjected before the.1am p is' put into rnormal service, ttor instance .soas toconvert .a smallfraction of the alkalineearth v.oxides which activatezthem intov alkaline-.earth metal.

The 'conditions .to which .the substance and products are subjected when.the lamp is beingmanufacturedfand which may influence the meltingpoints thereof are, for example, the j pressures ,to which ttheesubstance 'and V,prod- .ucts are subjected.

The products originating from said substance may'be temporary products, that is products which Wi1l.be:trans formed during later formationtoper-ations on the: electrode, as well .as ,permanent products, which ywill not .betrans- `formed any further.

In the known processes forfmanufacturing discharge lampsV the envelope :of .whichphas a very high silicacontent, `theactivated electrodes arefheated byrvarious methods. for .their formationbut all1these processesare relatively dithvcult 4to realize. For Ainstance, :heating aan @electrode tby flowing an'electric :current :through the Ymember which .supports '.the ,electroneemissive material necessitates ,two lead vwires `forvthe electrode, ibut .the 'manufacture-:of lead Wiresis diicult Whenxthey-must bezsealed vacuum-:tight in ,a vitreous materialhaving a lveryihigh silica content. Heating the supportrmem'ber :by discharges or by induced eddy'currents Vis also `d iicult, the inductiomfor example, requiring `magnetic elds 'dicult tofobtain inthetcase of electrodes the diameter .ofawhich is :smalband `which do Vnot compriseA ferromagnetic metal.

It is .among the .objectsrof the :invention-.tto providesa process .for manufacturingralampfof the type above .described, which process yis feasy .to use and `resultstmore regularly '.than .the known :processes iny satisfactory lamps.

In accordance with v the invention, 4the .-'substance 'from Which is obtained the electron-emissive material is :deposited on the support `member-for the emissivefmaterial in a condition of-suspensiondn ea.1iquid,and-the-rdegassing of .the alamp.. and lthe formation. of the .activated electrode, or electrodes,.,f the lamp. is .etected by heating the lWhole of .the lamp .to a .temperature higher-.than 900 '.C. while pumping the non-inert gases out of the lamp, said .elec- .trode, or. electrodes, being fat that .moment located. inthe .1.1amp.

The .pumping .of .the non-inert 1 gases .contained in .the -lamp may-.be...elfected.`.byf exhausting the lampfor byfflow- .ingcaninertgas through it, .or byzboththese means.

`Heating.thelamp .envelcpefabove .1300" C. is ygenerally to y.be .avoided :since vitreous ...silica .-devitries .at that temperature.

.In the process* of .the invention ,the .electrode .isheated .only .throughzthe lamp. envelope. There is .thenno need for heating theelectrode-.by anothenmethod,` morefdii- .cult to ,put .into practice, .or even impossible which. is .the -case of the owing `offadirect or low-,frequency .electric 'current vwhen the..electrodeiis.providedwith onelead-in -wire only. Thisprocess` is. easy. to use: .the .envelope may .be-heated l by ra, gas oven .or an .electric zeven, :or rby. one ,or more burners, .,or banlcsv of burners, using y.gaseous .or :liquid fuel.-

Fora bet-ten understanding ofthe invention and. .toshow .hoWlthe-same maybe .carried intoeect, electrodesfor which the processfof'thetinvention is' used, .as well Aas a ,lamp comprising such f zelectrodes, are represented in -the --accornpanying drawings, in which:

.'Fig. l represents diagrammaticall partly .in .-section, .partly sin elevation, `1an electrode fastened to t its ylead-.in

wire.

Figure 2 represents.diagrammatically, in .elevational view, a mercury vapor discharge lamp incorporating two electrodes of the kindsshownlinfigure l, the lamp being illustrated during manufacture at a point when these electrades have 1 just been introduced in the .'lamp .envelope Figures =3, i4, 5. represent diagrammatically,i in lsection, other forms of electrode.

In 'the electrodes lshown in Figure il, fthe metal `part carrying the emissive material .comprising altungsten'ro'd or pin'2,` of`l.51mm.in diameter,` andfa helixfl ofv tungsten Wire of OLS in diam-eter throughwhich the rod.2 has been passed. The helix l comprises l8.turnsiof .which vonly ten have 'been shown-for` greater .clearness ofJthe drawing. Theright! hand end-'olfI thehelix is` weldedielectrically to .the rod 4f2. 'The'.'spacing betweentwosa'djacent turns of'thehelix 31 is, onthe.average,.'about OLOStmm., the :clearance .betweenthe rod I2 iand the helix beingof the same order. ThecompositepartL .2 has undergone treatments 'intenderlfto purify .the tungsten surface,-well knownztreatments which involve, 'infp'articulan successive beatings to about l000 .C. in dry hydrogen andtthere after in'wet hydrogen.

Before applyingfto `the compositepart 1, 2=;a substance whichwvill be.transformedY into 1 an: emissive. material, such partisiweldedga few millimeters '.from :the left hand .end `o'ffthe helix,.to'.the\end iofia thinmolybdenumstrip .'.4fat the otherfend of 'which .a thickerfstrip117,*forvexample also kof molybdenum, lis -welde'dto forma lead-in lconnection' to 4theelectrode Vflllethinstrip 4,wliich is-.O-lS mm. thick, except atitsedges, rwhich'are thinned, isa few-centimetersilong- "-Itfis the sealing-offthe-strip #t to thef|:{uartzof'thel envelope 'f :the 'discharge device which will ensure -the f-vacuum tightness of current lead-in connection.

The assembly consisting df the part EIl,"f2, Ythe thin strip 4 'and the leadin 'connection 17 is -f'c-leanetl, for 'instance," by'heating inL hydrogen.

The part i1, i2 'is then coated, f'for instance by dipping or .byfmeans fof. a irbrnsh, Awith -'some v:calcium v.carbonate suspension, .this :coating being carried contain-.fsuchfa way as to ll, by capillarit-y,rthefclearances between thefturnsof the helix 1 and between said helix and the rod 2. The suspension is then allowed to dry in air, which yields a coat 3 of calcium carbonate. In some cases, the portion of the coat 3 which is on the outer surface of the part 1, 2 is removed with a brush, so as to avoid fouling the envelope of the lamp when its electrodes are introduced therein, if the electrodes then rub the envelope.

in a ball mill, for 60 hours, 100 grams of pure calcium carbonate, precipitated, with 100 grams of purified butyl acetate. After this crushing, about 250 grams of butyl acetate are added. With this dilution, the coatings 3 0btained are sufficiently adhesive although they contain no binder, and they contain enough calcium carbonate. Two semi-finished electrodes 6, 12 thus obtained are introduced into an envelope for a discharge lamp, made of quartz and comprising as represented in Figure 2, a central portion 5 provided with an exhaust tube 11 and two end pieces 8 and 16 which are open to allow the introduction of the electrodes therethrough. One electrode 6, is attached to molybdenum strips 10 and 9, and the other electrode 12, is attached to molybdenum strips 14 and 15. Following the introduction of the electrodes the two end parts are sealed by heating whereafter a vacuum is made through the exhaust tube 11, and the strips 10 and 14 are sealed to the end pieces 8, 16.

Once these two sealing operations are completed the lamp is connected through its exhaust tube 11 with a vacuum pump, and then placed in an oven which raises its temperature to about 1l00 C. This treatment, the duration of which is of the order of l5 minutes, results in a removal of gases from the electrode, and in a transformation of calcium carbonate into calcium oxide in acl cordance with the equilibrium reaction:

This reaction is complete in the direction which produces carbon dioxide, since it is carried out in a vacuum and for a sufficient time.

Once the electrodes have been so treated, there are introduced into the lamp, through the exhaust tube 11, argon and liquid mercury, whereafter the exhaust tube is closed by heating in the vicinity of the tube 5 in a known manner.

During these operations, neither the calcium carbonate nor its decomposition product, calcium oxide have melted. The pressure of the carbon dioxide in contact with coating 3 has always been very far from the high pressures at which it is possible to melt calcium carbonate or even the eutectic thereof with calcium oxide. The temperature reached during the heating in the oven is also very far from the melting point of calcium oxide (about 2580 C.). Since there has been no melting, neither the carbonate nor the calciumoxide have moved and have fouled the envelope of the lamp and formed excess thicknesses of emissive material at the expense of certain portions of the helix 1. The emissive layer is thus very uniform and electrodes manufactured in the same manner will have very similar characteristics.

High intensity discharges often used during the degassing and the forming of the electrodes are unnecessary with the above described process.

Another advantage is that it is possible to manufacture a lamp without any glass sealing after introduction of the electrodes, other than the closing of the exhaust tu'be and of the two end pieces.

The electrode for which the above numerical data is given is suitable, for example, for mercury vapor lamps operating with a vapor pressure of the order of two atmospheres, a current of several amperes, for instance 3.5 amperes, and a voltage gradient of 5 to 40 volts per centimeter length between electrodes when the inner diameter of the tube 5 is 25 millimeters.

The suspension is obtained, for instance, by crushing i Figure 3 shows, in section, an electrode, in which the metal coil forms two layers. The use of such an electrode, which is shorter than that represented in Figure 1, allows a better heating of the ends of the lamp during the functioning thereof. The coil 19, for example, comprises 15 turns. The outer diameter of the outer layer of the coil should be suiciently small to allow the electrode to pass through the end piece 8 or 16.

Figure 4 shows an electrode, the coil of which, made of tungsten wire 0.5 mm. in diameter with spacings of 0.05 mm., is divided into two portions: a front portion 20 which is not provided with electron-emitting material, and a rear portion 21, which supports an emissive coat. The portion 20 for instance, may comprise 14 turns and the portion 21 four turns, the distance between the portions 20 and 21 being about 2 millimeters. As indicated, the portion 21 only may be coated by dipping or by using a brush, with the substance which will be transformed into emissive material.

Figure 5 represents an electrode, the coil of which is divided into a front portion 22 with no electron-emissive material, and a rear portion 23 with a coil forming a double layer and having emissive material. The portion 22 may, for example, comprise six turns, and the portion 23 seven turns.

Regarding the activating substance suspension, it is also possible to use a suspension of calcium oxide, obtained, for example, 'by milling 100 grams of lime in grams of butyl acetate for 6 to 50 hours, then diluting the mixture obtained in 500 grams of butyl acetate. Thorium oxide may also be used. The heating by the oven, through the envelope, may then, in some cases, not be quite so strong as when calcium carbonate is used.

The suspension of activating material may also contain a refractory metal, tungsten for instance, in very nely powdered form, or silica, which may increase the adherence of the emissive material. A binder may also be added, for instance 1% in weight of nitrocellulose with respect to the activating substance.

Metal parts of the electrode may be made of molybdenum or tantalum, instead of tungsten. The coating with the substance which will produce the emissive material may be effected before welding the rod 2 to the thin molybdenum strip 4 of the lead-in.

Still other forms of electrodes may be used.

The discharge atmosphere may be krypton or xenon, with a pressure higher than 0.5 kg./cm.2 during operating.

What I claim is:

l. In a process for manufacturing an electric discharge lamp containing an ionizable discharge atmosphere and having at least one activated electrode, the discharge atmosphere having an absolute pressure higher than 0.5 kg./cm.2 during the operation of the lamp, the envelope of the lamp being made of a vitreous material having a very high silica content and said activated electrode comprising a refractory metal member provided with electron-emissive material, said electron-emissive material being obtained from a substance which has a melting point higher than 2500 C. and which produces, during the formation of the electrode, only products among which those which are not gaseous in the conditions of their production have melting points higher than 2500 C., these melting points being measured in the conditions to which the substance and products are subjected when the lamp is being manufactured, the steps of depositing said substance on said refractory metal member in a condition of suspension in a liquid, and of degassing the lamp and forming the activated electrode solely by heating the whole of the lamp to a temperature higher than 900 C. while pumping non-inert gases out of the lamp, said electrode being at that moment located in the lamp and the temperature reached by the envelope during this heating step being at least as high as the temperature reached by the electrode.

References Cited in the le of this patent UNITED STATES PATENTS Braselton July 9, 1935 Spanner July 15, 1941 

1. IN A PROCESS FOR MANUFACTURING AN ELECTRIC DISCHARGE LAMP CONTAINING AN IONIZABLE DISCHARGE ATMOSPHERE AND HAVING AT LEAST ONE ACTIVATED ELECTRODE, THE DISCHARGE ATMOSPHERE HAVING AN ABSOLUTE PRESSURE HIGHER THAN 0.5 KG./CM2 DURING THE OPERATION OF THE LAMP, THE ENVELOPE OF THE LAMP BEING MADE OF A VITREOUS MATERIAL HAVING A VERY HIGH SILICA CONTENT AND SAID ACTIVATED ELECTRODE COMPRISING A REFRACTORY METAL MEMBER PROVIDED 